Xsuite converter

[swhite2401]

This PR implements an Xsuite converter.

Xsuite is not required for this to work as everything is handled using json files.

This is still work in progress fro the moment, comments and suggestions welcome!

[lfarv]

Hello @swhite2401 : very interesting !
After a first look (even if it is still evolving), I have a few remarks:

• the BasicElement class should not inherit from Element: apparently none of the Element methods or attributes are used. And it's not meant to be used as an Element. So inheriting from object looks good enough.
• you could make use of the new Element.from_file class method:

    def get_at_element(self) -> Element:
        self._params_to_at()
        self._integrator_to_at()
        cls = self._class_to_at()
        return cls.from_file(self.atparams)
  

  Here, the _class_to_at method returns the AT class rather than storing it in atparams, and the _xsuite2at_class directories store AT classes rather than AT class names. It looks to me simpler, but your way is good as well.